Wireless communication using radio frequencies has become increasingly widespread in the last decade and many communication systems now compete for a limited resource. As a result, one of the most important parameters in the standards development for wireless communication systems is how efficiently a particular wireless communication system is able to use the allocated frequency spectrum.
The requirement for an efficient use of the scarce frequency spectrum resource has led to the development of wireless technologies that can operate with high levels of interference. For example, it is a key requirement for high capacity cellular communication systems that a high level of interference can be permitted. Typically these communication systems operate with a frequency reuse of one, which means that the same channel bandwidth is available and is used in all sectors and cells across the network. As a result, the intercell interference seen from the neighbour cells can be very substantial at the cell overlap areas. Since the power available to the transmitter is constrained, the available Carrier to Interference Ratio (C/I) and hence the data rate is also constrained under this condition. If the intercell interference can be removed, the effective C/I increases and the data rate increases commensurate with the improvement in C/I. This may provide a much higher spectral efficiency and increase the capacity of the system substantially, and it is therefore highly desirable to remove or mitigate the intercell interference.
A communication scheme which may be used in wireless communication systems is the Orthogonal Frequency Division Multiplexing (OFDM) scheme. Furthermore, a cellular communication system may use Orthogonal Frequency Division Multiple Access (OFDMA) wherein users in the same cell are assigned sub-carrier groups that are simultaneously active with other user's sub-carrier groups. However, in OFDMA, transmissions within a cell may be kept orthogonal and the interference generated to users in the same cell (intracell interference) can be effectively mitigated to the extent that it can typically be ignored.
Multicarrier communication techniques such as OFDM divide the total system bandwidth into a number of subcarriers. This is typically achieved by allocating symbols to subcarriers in a frequency domain representation of the signal to be transmitted, and then using an inverse fast fourier transform (IFFT) to generate the equivalent time-domain baseband signal.
Systems based on multicarrier modulation typically only allocate symbols to a subset of subcarriers, with the remaining subcarriers being left permanently unoccupied. The arrangement of subcarriers 100 in a conventional OFDM system is shown in FIG. 1. In this arrangement a number of subcarriers at the upper and lower edges of the frequency band are left unoccupied 115. These unoccupied subcarriers 115 can act as a guard band between this transmission and transmission on adjacent channels, as well as ensuring that any alias signals are sufficiently separated from the wanted signal to ease the filtering requirements in a practical implementation.
The subcarrier 105 corresponding to the direct current (DC) input to the IFFT is also usually left unoccupied. This ensures that the time-domain representation of the baseband transmitted signal has zero mean. Since the baseband signal does not contain a DC component, this then makes it much simpler for a receiver to estimate and remove any DC offset in the received signal.
Channel estimation in multicarrier systems is typically facilitated by transmission of pilot symbols, known to both the transmitter and receiver. Some systems transmit these pilot symbols on all non-zero subcarriers 110, whilst other systems are designed to only transmit pilot symbols on a subset of subcarriers (distributed in frequency). Once initial channel estimates are obtained for the subcarriers on which pilot symbols were transmitted, some systems may perform further processing on the channel estimates. This additional processing may improve the quality of the channel estimates and/or be used to obtain channel estimates for subcarriers on which pilot symbols were not transmitted.
It is known that DC offsets can be introduced by the transmitter and/or the receiver. Direct conversion (zero intermediate frequency (IF)) architectures for up/down conversion are particularly prone to the introduction of a DC offset. However, as long as the transmitted baseband signal is known to be zero-mean, an accurate estimate of the DC offset at the receiver can be obtained by simply estimating the mean of the received signal.
For multicarrier systems where the DC subcarrier has been nulled 100, this is easily achieved by taking the mean of the received time-domain signal 110 over an integer number of OFDM symbols (after removal of the cyclic prefixes). In theory, such a system does not need to estimate and remove the DC offset since no data is transmitted on the DC subcarrier. However, in a practical system the presence of a large DC offset would require support for a greater dynamic range in the baseband processing at the receiver. Furthermore, the presence of a large DC offset may also introduce additional signal distortion when combined with a carrier frequency offset. Therefore it is still beneficial to estimate and remove the DC component.
It is known that not all communication systems are designed to include a null on the DC subcarrier. Examples of such systems include receivers in single-carrier systems with frequency domain equalisation (SC-FDE) and the single-carrier frequency division multiple access (SC-FDMA) technique specified for use on the uplink of the 3GPP LTE standard. These systems can have a subcarrier arrangement 150, as also shown in FIG. 1, where the DC subcarrier 155 is not nulled and is used for the transmission of data.
For multicarrier systems that do not include a null subcarrier at DC, the impact from the introduction or increase of any DC offset. Firstly, the estimation of the DC offset by the receiver cannot use the same methods as systems that have a permanently nulled DC subcarrier, since the transmitted baseband signal does not have a zero mean. Secondly, unless the DC offset is sufficiently suppressed, it will introduce significant distortion to the data symbols being transmitted on the DC subcarrier, resulting in a direct degradation to link and system performance.
Consequently, current techniques are suboptimal. Hence, an improved mechanism to address the problem of DC offset estimation and compensation therefor would be advantageous.